The production of erythrocytes depends on the timely delivery of sufficient iron to erythroid precursors. The iron supply to the marrow comes under particular strain after hemorrhage, hemolysis, and other events that trigger expanded erythropoiesis. Both the absorption of dietary iron and the release of iron from stores are increased as erythropoiesis intensifies, but the mechanisms involved are not known, despite 50 years of research in this area. This competitive renewal application proposes and analyzes the mechanism by which erythropoietic activity influences iron homeostasis. Studies by our lab and others indicate that hepcidin is suppressed by factor(s) secreted by the bone marrow in response to erythropoietic stimulation, which then act on the liver. We therefore, initiated an unbiased search for a potential erythroid suppressor of hepcidin in the mouse by examining the time course of bone marrow gene expression after hemorrhage. We identified less than a dozen erythroid-specific transcripts whose expression changes prior to the suppression of hepcidin mRNA in the liver. Only one of them encoded a secreted protein, and this previously anonymous transcript (systematically named Fam132b and bearing resemblance to the TNF superfamily) was highly induced prior to hepcidin suppression. We have started analyzing the role of this transcript and the secreted protein using knockout mice, and have provisionally named it erythroferrone-1, as our preliminary data suggest it functions as a hormone linking erythropoiesis and iron metabolism. The specific aims of the proposed project are: 1. Define the role of erythroferrone-1 in erythron-dependent regulation of iron metabolism 2. Characterize the erythroferrone-1 protein and establish its mechanism of action 3. Determine the role of erythroferrone-1 in the pathogenesis of iron overload in -thalassemia Successful completion of the proposed studies would answer longstanding questions about a fundamental aspect of human and vertebrate biology. It may also provide a lead therapeutic candidate (a biologically-based hepcidin suppressor) for the treatment of diseases of hepcidin excess such as anemia of inflammation or iron-refractory iron deficiency anemia (IRIDA). If the same or related factors also cause the suppression of hepcidin in -thalassemia and other iron-loading anemias, the work will provide important insights into the pathogenesis of iron overload in these globally common diseases. Pharmacologic antagonists of newly identified suppressors of hepcidin could offer new treatments to control the lethal iron overload in -thalassemia and other iron-loading anemias.